Welcome to Practical Kotlin Deep Dive. This book is designed for Kotlin developers who want to deep dive into the Kotlin fundamentals, internal mechanisms, and leverage that knowledge in their daily work right away.

Each topic provides in-depth explanations that go beyond surface-level knowledge, exploring not just how Kotlin features work but why they were designed that way. You’ll find references to KEEP (Kotlin Evolution and Enhancement Process) proposals, JetBrains YouTrack discussions, and compiler source code, giving you direct insight into the design decisions behind the language.

This book includes relevant resources and additional references for those seeking further study beyond its content. Efforts have been made to provide footnotes for key terms that may be unfamiliar or complex, ensuring that developers at all levels can grasp challenging concepts while deepening their understanding.

The “Pro Tips for Mastery” sections dive deeper into advanced topics, uncovering internal structures of the Kotlin fundamentals, and offering expert insights that you can apply immediately to write better code. These sections bridge the gap between knowing a feature exists and truly understanding how to use it effectively.

This book covers Language Fundamentals, Standard Library, Coroutines, Compiler and Plugins, and Kotlin Multiplatform in a structured progression. However, it is not intended to be an exhaustive reference for every aspect of Kotlin. Instead, it focuses on practical knowledge that makes a real difference in how you write, debug, and optimize Kotlin code.

Best of luck on your journey to mastering Kotlin!

This is Jaewoong Eum (also known as skydoves1), a Google Developer Expert (GDE) for Android, Kotlin, and Firebase. I’ve created over 80 open source libraries and projects, which collectively achieve more than 15 million downloads annually by developers worldwide. I’m also the founder of Dove Letter2, a subscription based repository where I share, explore, and discuss topics around Android, Jetpack Compose, and Kotlin.

With a mission to make the world a better place and create a positive impact through technical solutions, open source contributions, and technical content, I believe this book marks another exciting chapter in this journey following my first book, Manifest Android Interview3. I want to express my deepest gratitude to the developer communities, friends, and family who have supported me along the way. Writing this book is not just a milestone but the first step toward empowering others in the Kotlin ecosystem.

I hope this book helps you gain new perspectives, sharpen your problem solving skills, and build a comprehensive understanding of Kotlin, from the language fundamentals to advanced topics like coroutines, the compiler, and multiplatform development. Learning should always be an ongoing journey, and every challenge you overcome makes you a better engineer. Whether you’re preparing for your next technical interview or simply pushing yourself to master Kotlin, I encourage you to think beyond implementation: understand the why, experiment with different approaches, and continue learning with curiosity and passion.

As always, happy coding, and best of luck on your Kotlin journey!

Chapter 0. Kotlin Language

Chapter 1. Kotlin Standard Library

Chapter 2. Coroutines

Chapter 3. KotlinX Libraries

Chapter 4. Kotlin Compiler and Plugins

Chapter 5: Kotlin Multiplatform